1. Technical field of the Invention
The present invention relates to a multi-mode ground surveillance airborne radar. It applies notably to the field of synthetic antenna radars aboard airborne platforms, or SAR radars according to the acronym standing for the expression Synthetic Aperture Radar, and more particularly to devices of this type intended to produce a large-swath image of the ground, according to the so-called STRIPMAP technique, from a safety distance from the terrain overflown.
2. Discussion on the background
A known application of synthetic antenna or SAR radars is the capture of images by successive strips, designated by the name STRIPMAP. According to this technique, an airborne platform, also dubbed a carrier, typically an aircraft, is equipped with a radar antenna, making it possible to construct an image in the form of a band, as the carrier advances, by juxtaposing a series of strips. The image of a whole area of interest can thereafter be constructed by juxtaposing all the bands. The aim of the radar antenna on the carrier is generally sideways-looking, and the capture of image strips is commonly carried out at an angle 90° to the axis of the carrier's velocity vector.
The image reconstructed by juxtaposing strips is thus a band whose width equals the width of a strip. This width is usually designated by the term swath. The length of the band is solely limited by the duration of selection of the radar mode as the carrier advances. Thus, the duration taken to obtain as output from the system a band of length X with a carrier the modulus of whose velocity is V, is equal to the time taken by the carrier to travel the distance X, i.e. X/V. For example 200 seconds are necessary for a carrier moving at a speed of 100 m/s, to produce an image over a band of a length of 20 km.
During these 200 seconds, the radar is mobilized full-time. Thus it can operate only in the STRIPMAP mode. For example it can provide information about the fixed echoes situated in the area covered by the band, but not about the moving echoes of targets moving around in this same area.
There exist radars known from the prior art, making it possible to provide multiple information about static targets and moving targets. These devices are based on combined SAR and GMTI modes, the acronym GMTI standing for the expression Ground Moving Target Indication, and being commonly used in the technical field of radars. However, radars currently capable of affording such functionalities are the most efficacious and the most expensive radars. The latter may notably employ two-plane electronic scan active antenna technologies coupled with extremely high calculation powers, so as to provide interleaved radar modes which in one and the same temporal sequence, extract the moving and fixed targets situated in a given area. These very sophisticated devices require several waveform generating systems and several receivers, and very significant calculation power. Furthermore, the carrier platform must be compatible with the constraints related to systems of this type, and must therefore notably be capable of stowing a significant volume onboard; the impact on fuel consumption of the carrier platform is also significant.
Stated otherwise, the existing devices allowing multi-mode radar operation, notably capable of ensuring the tracking of fixed and moving targets, are reserved for big, inflexible and relatively expensive systems.